http://www.webbikeworld.com/motorcycle-helmets/motorcycle-helmet-faq.htm
As this article points out, there is more than just size to a good fit. Shapes vary and this is important for comfort and a good fit. I've bought my last two helmets online with no problems. The manufacturer typically gives you a chart suggesting S, M, L, XL and so forth in their product line, based on the measurement around your head and the specific helmet model you want. In other words, just because you may have a 56 cm measurement, some of their models may be a S and some others may be a M.
Hope this helps some. Obviously, if you can try the helmet on first this is best, but when this is not possible, you can usually return for a different size when purchased online.